Foon is a ZX-Spectrum emulator for the Game Boy Advance.
The ZIP file contains a Windows executable. Run this executable, and drag your .Z80 files from Explorer into the large white area at the top of the dialog box. You will see games being added to the list. Double-click on an entry to edit the key mappings for that game. You can map any GBA button except START to any Spectrum key or Kempston Joystick motion.
The defaults for each game are basically Kempston joystick motions. If the game requires you to, for instance, “Press 0 to Start”, then don’t forget to assign one of the spare buttons to “0”. There will eventually be support for dynamically generating unassigned keypresses at runtime, but not today.
Once you’ve assigned all the appropriate keys, select an output file, and hit the Create Image! button. This will generate a .BIN ROM image file that you can transfer to your GBA. Note that the Injector doesn’t fix up the GBA rom header.